197 Aerodynamics (CFD analysis)

[Zuban]

Ah, ok. Realizable k-epsilon is what I use in fluent, with Non-equilibrium wall functions.

Hour or two? That's fast! What type of mesh and what size are you running? Open foam is up there with the best CFD software available from what I've heard so it should give accurate results if set up right.

I will need to go back over my setup and check, its been about a year since i did anything with it, and have since reorganised my lab setup, and managed to break my source ubuntu vm I set everything up on... but openfoam does seem to be good, but its not easy to use, and because my understanding of cfd is basic, its a vertical learning curve... I was using it really to see whether changes I made had the desired effect or not, positive or negative, and from that point of view it does the job.

From memory whatever setup i had was memory limited, if you set the mesh detail too high then you wont get an error, you will just use more ram than the system has, and end up with the process using paging or swap file instead, and then your performance drops through the floor as instead of ram which is fast, part of your sim is being run from disk. So I always tune the setup to make sure its within physical memory.

 

[tomislavp4]

The simulation on the Sprint spoiler just got completed. It's effects are similar to those of the Cup pack but with lesser magnitude. It increases the drag by 12,3 percent and the lift by 31,1 percent so the improvement of the lift to drag ratio ends up being 16,5 percent.

Notice that I've added a column in the spreadsheet "fDF" as in "front downforce" which shows how big of a percentage of the total downforce acts on the front axle. The stock car has 60 percent of the downforce on the front axle while the Cup has 60 percent on the rear axle. The car with the Sprint spoiler has downforce that is evenly balanced between the axles. I've also added a Vmax column to show how how the changes affect the top speed. Values are in kilometers per hour so you might want to convert them to whatever you feel comfortable with.

Here are velocity and low pressure plots on the symmetry plane for the Cup and the Sprint spoiler. Not much of a difference between the two other than the slight change in the wake. The Cup spoiler seems to be helping the diffuser suck a bit more air so the pressure in the diffuser area is lower on the Cup car, thus the difference in downforce. If I was to choose between the Cup and the Sprint spoiler based only on the results from these simulations I'd go with the Cup. It is more efficient at producing downforce which is what we want from a spoiler.

 

[tomislavp4]

Damn it. You can disregard the Sprint spoiler results for now. I'll have to run it again as I've used an older model with slightly different front bumper.

 

[petercronin]

@petercronin @Dmonk197 there's conflicting information about that. I've read both 40 kg and 70 kg for the df produced by the diffuser but the speed was not mentioned. Do you have a link? As this most likely is the df that the diffuser produces it should be subtracted from the lift that the rest of the car produces to get the net down force.

I don't but from memory I think it was either in the official Renault sales brochure or it was something somebody from RUK stated on the renaultsport.com forum. I'm almost certain it said 40kg at 80mph though and yes I think it was referring to the diffuser.

 

[///M]

Oh man, you got me all excited and then said to disregard it!



Noticed that the Cup model has the splitter too, would that affect other bits?

As for your thoughts on a custom splitter, I'm looking at it exactly the same. You'll be an absolute HERO to me if you come up with something that works, looks good and could be made.

I have a pretty solid idea for how I'd like it to look but my skills don't go any further than photoshop! But yeah, something more like that Porsche you posted would be so much better.

 

[tomislavp4]

Oh man, you got me all excited and then said to disregard it!

Sorry mate, my bad!

Noticed that the Cup model has the splitter too, would that affect other bits?

Yeah, the splitter should affect things so I ran a simulation on the Cup spoiler without the splitter to see how much of a difference it does make. Turns out it doesn't do much. So while the spoiler is functional, the splitter is all show and no go, at least according to these simulations.

Here are the values for all the simulations I've done thus far, including the sprint spoiler on the correct model. The Cup spoiler does have a slight edge when it comes to downforce and efficiency so if it's performance you're after, that one's for you. If you prefer the looks of the Sprint though, it's fine, you still get some increase in downforce and the reduction in top speed is lower compared to the Cup one.

The pressure plots show how similar the Sprint and Cup spoilers are. They are nearly identical.

 

[///M]

Brilliant mate. I love how you can see the areas of pressure in the images. Very funny about the splitter. Are you running it at stock ride height for now? I'd be really interested to see if lowering it a lot made any difference to how effective (or not) the splitter and the diffuser are.

I bet the side skirts don't do much either. They look good though!

Most of all I'd love a proper splitter design on there though! Always had a feeling that splitter did nothing!

 

[tomislavp4]

Brilliant mate. I love how you can see the areas of pressure in the images. Very funny about the splitter. Are you running it at stock ride height for now? I'd be really interested to see if lowering it a lot made any difference to how effective (or not) the splitter and the diffuser are.

I bet the side skirts don't do much either. They look good though!

Most of all I'd love a proper splitter design on there though! Always had a feeling that splitter did nothing!

Stock ride height for now. I do plan on doing som runs on lowered/raked models though so stay tuned. The side skirts don't do anything and I don't need CFD to tell me that!

 

[///M]

Stock ride height for now. I do plan on doing som runs on lowered/raked models though so stay tuned. The side skirts don't do anything and I don't need CFD to tell me that!

Yeah but in all the Renault marketing they showed how they helped the airflow and stuff...

 

[Zuban]

do you have a small gap between the cup spoiler and the rear bodywork? be interested to know how much effect that gap has, and how you can change it to alter the wings effect. I played around with this a little and noticed it had some effect, it interested me because the potential to alter the wing mounts is something that is achievable, I also played around a little with changes to the end plates, enlarging them downwards and backwards and that had some interesting effects on drag if I remember correctly.

 

[tomislavp4]

do you have a small gap between the cup spoiler and the rear bodywork? be interested to know how much effect that gap has, and how you can change it to alter the wings effect. I played around with this a little and noticed it had some effect, it interested me because the potential to alter the wing mounts is something that is achievable, I also played around a little with changes to the end plates, enlarging them downwards and backwards and that had some interesting effects on drag if I remember correctly.

Yes, there is a gap between the edge of the bodywork and the spoiler but since I do not actually have these things laying around and modell from pictures, I cannot guarantee the accuracy of the size of the gap. Increasing the size should make the thing act more as a wing and less as a spoiler, which should give more downforce because then you'll have an airfoil in free stream. What you say about the endplates makes sense, they should affect the drag coefficient. By enlarging them you effectively make the sides of the car longer which does reduce the wake and thus the drag.

 

[tomislavp4]

Alrighty, the simulation on the Maxton stuff is finished. It has a smaller impact than both the Cup and the Sprint parts with a 7% increase in air resistance and 22% increase in downforce. What it has going for it is that it is the most balanced, with almost 50/50 downforce distribution between axles. It doesn't affect the top speed as much as the other two either.
Here's a pressure plot on the symmetry plane for the Maxton and the Cup pack model. Notice how the Cup creates an area of lower pressure behind the car compared to the Maxton. This lower pressure area helps suck air through the diffuser creating lower pressure under it but it also increases drag. Up front though, it's the opposite. The Maxton splitter seems to be better at creating low pressure under the car.

 

[tomislavp4]

So I got a 2Gb RAM module from a friend that no longer needed it and decided to run everything again, with slightly increased accuracy. The numbers came out slightly different but as you can see from the table below but the plots are virtually unchanged so I wont be posting new ones. I also did simulations on a car with opened side vents, a car lowered by 30 mm as well as a car lowered 40 mm at the front and 20 mm at the rear.

The simulation on the car with opened side vents shows that opening the vents does increase downforce slightly. The interesting thing is that the improvement in downforce comes mainly from the rear of the car, not the front. Now I've been trying to explain this and the best I can come up with is that opening the vents reduces the air going under the car from the wheel well but I haven't been able to show this. Here's the pressure within the wheel well. Almost no difference whatsoever. And this is the where the air flowing through the vent comes from and where it goes.

Now it is expected that the area within the wheel well is more affected by the limitations of the simulations which were discussed earlier in the thread so these results should be taken with more salt than usual.

Moving on to the lowered models, they both give lower drag and more downforce compared to stock. The raked model is superior to the one that is lowered by the same amount both front and rear so if anyone is thinking of lowering their Clio for performance reasons, that's how to go about it. Here's a low pressure plot on the symmetry plane for the stock and raked models. Notice that the raked model moves the downforce balance slightly to the front. Lowering the car by the same amount also does this but to a lesser degree.


And here's the table. I've changed it again to make it more readable and less messy. The downforce values are for speed of 30 m/s (108 kph or 67 mph) to get values for double the speed, multiply them by 4.

 

[Zuban]

superb work! are your downforce figures straight downforce? or is this lift reduction, would have thought it unusual the stock car would produce any downforce, although most cars don't have a working rear diffuser as standard lol

 

[///M]

Another well put together, really interesting post mate, thanks a lot. I still don't understand a lot of it but it's a damn good read all the same!

 

[R32pete]

Ah makes sense about the rake cosmetically less pleasing but makes the car pointy too.

I like what this guys done on bumper the rear of the wheel, do you think it does much in the way of reducing drag etc?

 

[tomislavp4]

superb work! are your downforce figures straight downforce? or is this lift reduction, would have thought it unusual the stock car would produce any downforce, although most cars don't have a working rear diffuser as standard lol

Downforce, not lift reduction. And yes, the DF values are optimistic. Like I wrote earlier, simulations of this kind overestimate the coefficient of lift. Now according to PhD studies I have, regular hatchbacks and wagons are closer to neutral (zero lift) compared to sedans, and they can easily gain downforce with the addition of a diffuser. Here's an example
You can see how the stock car has a slight downforce with a lift coefficient of -0,028 and that adding a diffuser increases the downforce to -0,12. So according to this, we can expect the Clio to have a similar, maybe slightly better lift coefficient when accounted for engine bay airflow. So a good idea is to take the values in my table and divide them by 2, that'll bring you closer to the real value. Of course, real values don't matter here because real values isn't what we're after in the first place. What we're after is the change between different configurations.

 

[tomislavp4]

Ah makes sense about the rake cosmetically less pleasing but makes the car pointy too.

I like what this guys done on bumper the rear of the wheel, do you think it does much in the way of reducing drag etc?

View attachment 19671

No idea mate. I'd guess not but I can test it.

 

[tomislavp4]

Right. I've started doing simulations on custom parts, first one being a simple splitter. The picture below shows how it looks. It's basically a flat piece, 15 mm thick, that starts just below the rubber lip of the bumper so it makes the front lower by 15 mm. It extends 80 mm forward from the bumper for most of its width.

And here's how the car looks with it. Pretty good if you ask me.


All simulations done on custom parts will be done with the vehicle lowered by 40 mm at the front and 20 mm at the rear because that's the most promising of the "stock" configurations. The results show a 33% increase in downforce compared to stock and around 8% compared to the raked model. The drag is slightly lower than stock and slightly higher than raked but the difference is negligible. Front downforce is increased by 31% compared to the raked one but the rear downforce is decreased by 23% so the downforce split is 79/31 front to rear and needs balancing out with the help of a wing or a spoiler, as expected.

Here's the obligatory symmetry pressure plot, showing the splitter doing what it's supposed to.


And here's the table.

 

[///M]

How to make my heart sing and then break it all in one post!!!



The moment I saw that splitter... Everything I've always wanted for the 197! Looks so good. But my god what a difference it makes! Seems a huge difference and as Mrs M is adamant we're staying spoilerless, it looks like we're staying splitterless too!

Would be interested to see the splitter in conjunction with a Cup spoiler. And maybe a custom ducktail one too

Excellent work as always mate.